1. Field of the Invention
The present invention relates to a money identifying method and a device for identifying money on the basis of a surface image of the money, and more particularly to a money identifying method and a device which improves its money identification accuracy by determining a binary threshold value based on a density histogram of a predetermined part of the money to optimize binarization of the money.
2. Description of the Related Art
Conventional coin identifying devices for identifying coins are comprised of one or more magnetic sensors disposed on a coin passage through which coins inserted through a coin insertion port are moving, to determine a material, thickness and size of each coin on the basis of detection output from the magnetic sensor(s) and to identify the inserted coins as authentic or counterfeit and denominations according to the determined results.
But, coin identifying devices having the prior art magnetic sensors have a problem in that identification of a coin as authentic or counterfeit and denomination has become more difficult because the accuracy of making an altered coin, which is a foreign coin similar in material and shape to a domestic authentic coin, has improved.
In order to detect such altered coins, it becomes necessity to identify coins with higher accuracy. It is proposed to use a surface pattern of each coin as an identification element for identifying the coins.
For example, Japanese Patent Application Laid-Open No. 8-16871 discloses a method which is comprised of irradiating light to a surface of a coin, detecting a reflected light by an area sensor, converting the detected image into an electrical signal, performing a binarization processing of the converted image signal, and identifying the surface pattern of the coin from the binary image.
Japanese Patent Application Laid-Open No. 9-231432 discloses a method which is comprised of irradiating light to a surface of a coin, detecting a reflected light by an area sensor, converting the detected image into an electrical signal, detecting a material of the coin by a magnetic sensor, identifying denomination of the coin, extracting an effective image area according to the identified denomination of the coin, determining a binary threshold value of the extracted image area, performing a binarization processing of the image signal of the coin surface, and identifying the surface pattern of the coin from the binary image.
But, the aforesaid prior technologies disclosed in Japanese Patent Application Laid-Open No. 8-16871 and No. 9-231432 have disadvantages in that, because the binary threshold value for binarizing a two-dimensional image is variable depending on brightness and contrast of the original two-dimensional image, it is necessary to adjust a method of calculating the binary threshold value according to a state of the image, and in order to calculate an effective binary threshold value, it is necessary to calculate variance between classes with respect to the respective binary threshold values by sequentially changing the threshold value within a predetermined range, and it takes significant time to calculate the binary threshold value. Thus, it was difficult to speed up the coin identification.
Where a two-dimensional image of an uneven surface pattern inherently not having a large difference between light and shade portions of a coin is binarized, if an unnecessary area such as a background of the image (part where the coin is not imaged) is included in the image area, the binary threshold value calculated from the image area is not a binary threshold value effective for identifying the uneven surface pattern of the coin but a binary threshold value effective for determining a boundary between the periphery of the coin and the background of the image, and it becomes difficult to identify the uneven surface pattern of the coin.
Such problems are not limited to the identification of coins but also apply to the identification of money performed by obtaining a surface image of money such as paper money and binarizing the surface image.